1. Field of the Invention
The present invention relates generally to integrated thin-film photoelectric conversion modules and particularly to those with bypass diodes.
2. Description of the Background Art
Generally when a plurality of thin-film photoelectric conversion cells are connected in series to form an integrated thin-film photoelectric conversion module, a plurality of elongated rectangular cells are integrated in a direction of a minor axis thereof (see for example FIG. 4A and an associated description provided hereinafter). If dead leaves, bird-droppings and the like stick and cause shadows on a light receiving surface of a cell in such a module, the cell""s photoelectromotive force decreases and then the module""s overall output significantly decreases, because the cell with the reduced photoelectromotive force behaves as a diode connected in series in an opposite direction to a generated electric current and it thus exhibits an extremely large resistance.
To alleviate such a disadvantage, a plurality of cells connected in series are divided in parallel to form a plurality of series arrays which are in turn connected in parallel, as disclosed for example in Japanese Patent Laying-Open No. 2001-68713. Then, even if a cell in a series array has a photoelectromotive force reduced to zero, it would not impair electric current in the other series arrays connected in parallel to that series array and thus overall output of the module can be prevented from being significantly reduced.
If the cell behaving as the diode receives a reverse voltage more than its breakdown voltage, however, a local dielectric breakdown occurs at a portion having a lower withstand voltage. Electric current does not flow uniformly in the cell having suffered the local breakdown and thus causes local heat generation referred to as xe2x80x9chot spot phenomenon.xe2x80x9d
If the module includes a thin-film not firmly adhering to a substrate, such heat generation causes an impaired appearance of the portion having suffered the local breakdown, but such does not significantly impair reliability of the module when small current flows through the cells. However, since a module having a large area generally outputs large electric current, local large current flows in the cell having suffered the local breakdown. As a result, a metallic electrode layer in the cell melts and the entirety of the cell can be destroyed in the end.
A well known approach to avoid such a disadvantage is to connect bypass diodes each in antiparallel to at least one of a plurality of photoelectric conversion cells connected in series. More specifically, if a photoelectric conversion cell is shaded, a bypass diode connected in antiparallel to the cell can serve to pass electric current generated in the other cells connected in series to the shaded cell. In other words, as a threshold voltage in a diode formed of a thin-film is about one tenth of its reverse breakdown voltage, reduction of output in a photoelectric conversion module can thus be made very small.
U.S. Pat. No. 6,013,870 discloses forming a bypass diode in the same layer structure as a thin-film photoelectric conversion cell formed on a main surface of a substrate. It describes that an uneven boundary of a front electrode layer is formed between a cell and an adjacent bypass diode, that a protruding edge portion of the front electrode layer of the cell overlaps and is connected to a back electrode layer of the diode, and that a protruding edge portion of the front electrode layer of the diode overlaps and is connected to the back electrode layer of the cell. Then, the diode can be connected to the cell in antiparallel.
In this method, however, the photoelectric conversion cell and the diode are formed adjacent to each other and thus there is a possibility that formation process of the diode may damage the cell. Further, since the method requires forming the boundary pattern having small protrusions and depressions in the electrode layer and the patterning is time-consuming, the method is not suitable for practical production. Furthermore, there is a serious problem in the method that a xe2x80x9creverse bias treatmentxe2x80x9d of cells cannot be carried out, as will be described hereinafter.
Regarding an integrated thin-film photoelectric conversion module having a large area, it is generally known that conditions of depositing thin layers, patterning the layers with a laser beam for integration, and the like are likely to cause local short circuit defects between first and second electrode layers sandwiching a semiconductor layer and thus likely to cause insufficient output characteristics in the module. Accordingly, for example, Japanese Patent Laying-Open No. 10-4202 teaches to burn out and remove the local short circuit defects, wherein voltage reverse to output voltage of the cells is applied between the second electrodes of adjacent cells connected in series. This is generally referred to as a xe2x80x9creverse bias treatmentxe2x80x9d of a cell.
In a case that series-connected thin-film photoelectric conversion cells each with a bypass diode connected in antiparallel thereto are integrally formed on a substrate as in U.S. Pat. No. 6,013,870, if a cell is to be supplied with the reverse voltage in the reverse bias treatment, then the bypass diode would receive forward voltage. More specifically, the bypass diode receives forward current flowing therethrough in the reverse bias treatment of the cell and thus sufficient voltage would not be applied to remove the short circuit defects in the cell. In such a case, if large voltage is forced to be applied to the cell, then the bypass diode would be destroyed by excessively large forward current flowing therethrough.
In general, an integrated thin-film photoelectric conversion module has a plurality of thin-film photoelectric conversion cells connected mutually in series on a glass substrate. Each cell is formed by depositing a front transparent electrode layer, a thin-film photoelectric conversion unit and a second electrode layer on the glass substrate and subsequently patterning the layers for integration.
For such an integrated thin-film photoelectric conversion module, there still exists a demand for improved photoelectric conversion efficiency. A tandem structure includes a stack of a plurality of thin-film photoelectric conversion units each having a different absorption wavelength range between a front transparent electrode and a back electrode, and it is known as a structure capable of photoelectricly converting incident light in improved efficiency.
One type of tandem structure is a hybrid structure, which includes a stack of a plurality of photoelectric conversion units including photoelectric conversion layers having different crystalinities. For example, a thin-film photoelectric conversion unit closer to a light receiving side (or a front side) includes a photoelectric conversion layer formed of an amorphous silicon layer having a wide bandgap and a thin-film photoelectric conversion unit closer to a backside includes a photoelectric conversion layer formed of a polysilicon layer having a narrow bandgap.
For material such as silicon, a crystalline layer formed on a substrate for example through plasma chemical vapor deposition (CVD) contains residual stress much greater than that in an amorphous layer. Residual stress of a film counteracts adhesion of the film to a base. More specifically, apparently measurable adhesion of a thin-film to a base is the true adhesion at the interface minus the effect of the residual stress of the film. In an integrated hybrid thin-film photoelectric conversion module, therefore, effective adhesion of a semiconductor layer to a base is reduced and then the reduced effective adhesion further notably impairs an impaired appearance of a portion having suffered dielectric breakdown due to the aforementioned hot spot phenomenon.
In view of the prior art as described above, an object of the present invention is to provide an integrated thin-film photoelectric conversion module capable of achieving high output and high reliability and particularly to provide a highly reliable integrated hybrid thin-film photoelectric conversion module fabricable rather simply and inexpensively.
According to an aspect of the present invention, an integrated thin-film photoelectric conversion module includes a multi-layered film including a first electrode layer, a semiconductor layer and a second electrode layer stacked on a main surface of a substrate. The multi-layered film includes a cell region including a plurality of photoelectric conversion cells connected in series, a bypass diode region and a connection region. When the cell is subjected to reverse bias treatment, the connection region does not connect the bypass diode to the cell. After the cell is subjected to the reverse bias treatment, the connection region connects the bypass diode in antiparallel to at least one of the cells connected in series.
Preferably, the connection region has the first electrode layer and the second electrode layer short-circuited to enable the reverse bias treatment of the bypass diode. Further preferably, before the reverse bias treatment of the cell, the connection region has the first electrode layer and the second electrode layer short-circuited by a conductive material applied in a gap extending from the second electrode layer to the first electrode layer.
In the reverse bias treatment, on the other hand, connection of the connection region to the cell is preferably cut by a gap penetrating the multi-layered film.
Preferably, selected one of the cells connected in series has the second electrode layer continuous to the second electrode layer of the connection region and also connected to the first electrode layer of the connection region by a conductive material applied after the reverse bias treatment to fill the gap penetrating the multi-layered film. The connection region has the first electrode layer continuous to the first electrode layer of the bypass diode. The bypass diode has the second electrode layer continuous to the second electrode layer of the cell different from the selected cell.
In the cell region, each of the cells can have an elongated rectangular geometry and the cells can be connected in series in a direction of the minor axis thereof. Gaps can be provided between the cell region, the bypass diode region and the connection region for adjusting electric connections. Each of the gaps can be formed in linear line segment parallel to the minor or major axis of the rectangular cell.
One or two of the bypass diodes can be provided adjacent to one end or both ends of the major axis of the cell. Further, the cell region can include more than one array of the cells connected in series, the arrays being connected in parallel, between two adjacent the arrays there being arranged the bypass diode connected in antiparallel through the connection region to both the same numbers of the cells in the two adjacent arrays.
The module can include the semiconductor layer including an amorphous photoelectric conversion layer and a crystalline photoelectric conversion layer arranged in tandem.
According to another aspect of the present invention, a method of fabricating the integrated thin-film photoelectric conversion module, includes the steps of applying reverse bias voltage between the second electrode layer of one of the cells and that of another cell adjacent to the one cell or between the second electrode layer of the bypass diode and that of the connection region adjacent to the bypass diode to eliminate short circuit defects in at least one of the cell and the bypass diode, and then applying conductive material in the gap penetrating the multi-layered film.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.